Personal computers (PC's) utilize floppy disks for transferring data to and from the computer. Floppy disks encoded in modified FM (MFM) are in widespread use. It is necessary to synchronize the transfer of information from the disk in the disk drive of the computer with the internal clock of the computer. To accomplish this, the voltage controlled oscillator of the computer, which is used to provide the clock signals, is synchronized in frequency with the synchronization field frequency encoded on the disk. It is possible for variations as much as plus and minus 10% to occur between the internal clock frequency of the computer and the synchronization field frequency read from the disk. This may be caused by differences between the speed of operation of the disk drive motor in the computer where the information originally was stored and the motor in the drive of the computer with which the disk is to be used.
In the past, this synchronization typically has been handled by voltage controlled oscillators in conjunction with an analog phase-locked loop (PLL). Such analog phase-locked loops employ a re-triggerable monostable multivibrator or "one-shot", which is continuously triggered into its astable mode by each incoming synchronization field bit. If these bits do not occur successively prior to the time-out period of the monostable multivibrator, it resets to its stable state, indicative of a failure to detect a synchronization field frequency. The detection of the synchronization field frequency is necessary to enable the PLL loop to respond properly to incoming data for synchronization purposes.
It has been believed that only an analog system could provide the necessary reliability. Three significant disadvantages, however, are inherent with analog monostable multivibrator circuits for providing the detection of synchronization field frequency encoded on the disk. The first of these disadvantages is that such circuits are difficult to fabricate with precision and predictability in an integrated circuit. Generally, analog monostable multivibrators require external trimming components, in addition to those components which can be fabricated directly on the integrated circuit.
A second disadvantage is that analog components are subject to significant drift with temperature. Thus, any analog design must allow sufficient margin for such drift.
The third, and perhaps most significant, disadvantage is that the resulting frequency detector necessarily detects all frequencies which are greater than or equal to the desired one (namely, the synchronization field frequency). Since this has been necessary with past analog circuits using a monostable multivibrator as the detector, the frequency sought (and which is used on the floppy disk as the synchronization field) must be the highest frequency encountered. If the frequency, however, is too high, the monostable multivibrator cannot distinguish this condition, since any frequency equal to or greater than the frequency of interest will cause the output to be indicative of detection of the desired frequency.
A digital synchronous field detector circuit has been implemented to replace the conventional analog circuit composed of a monostable multivibrator. This digital synchronous field detector employs eight phase window pulses generated by the reference clock and classified into four pairs; so that the difference in phase of each pair is four times the period of the reference clock. A pattern of synchronous fields then is detected whenever two neighboring "ones" in the input data exist in paired window. The window pulses generated from the reference clock are asynchronous with the read data; and they have different frequencies and phases. The read data read from a disk includes significant jitter. Consequently, in order to secure operation, whatever the phase relation, an arrangement is made in this system so that the windows are continuous, and neighboring windows overlap. The result is a very complex circuit, requiring a large number of components. The system, however, does overcome the adjustment and temperature drift problems inherent in conventional analog circuits.
It is desirable to provide a digital frequency detection circuit which is suitable for detecting the digital synchronous field of an MFM encoded floppy disk which is simple, uses a minimum number of components, and is capable of detecting frequencies within a range of frequencies determined by a lower frequency limit and an upper frequency limit, such that the frequency detection circuit overcomes the disadvantages of the prior art noted above.